1. Field of the Invention
The present invention relates to a method of polarization-treating a piezoelectric body for use in a ceramic filter, a ceramic oscillator, and other suitable components.
2. Description of the Related Art
For polarization-treatment of piezoelectric ceramic substrates (block, unit, or other suitable substrates) of PZT and PT types, conventionally, after a piezoelectric ceramic substrate is fired, electrodes made of Ag or other suitable materials are provided on both of the opposite sides of the piezoelectric ceramic substrate. Plural piezoelectric ceramic substrates are simultaneously dipped into a polarization liquid at a temperature of 60 to 100xc2x0 C., and a voltage of 2 to 8 kV/mm is applied for about 10 to 30 minutes to obtain a desired polarization degree.
After the polarization treatment, to prevent the deterioration of characteristics caused by heat treatment as an after-process, the piezoelectric body is left to stand still in an atmosphere at about 150xc2x0 C. for 20 to 30 minutes (aging), so that the characteristics of the piezoelectric body are caused to deteriorate, and thereby, the time-dependent characteristics of the piezoelectric body are stabilized.
The polarization treatment of a piezoelectric body can be done by the in-liquid polarization which is carried out in a polarization liquid having insulation properties as described above, and the in-air polarization which is conducted in the atmosphere or in an gas atmosphere. In the in-air polarization, a desired electric field intensity cannot be attained because discharge occurs at a voltage of about at least 1 kV/mm. For this reason, in general, the in-liquid polarization is used to obtain a high polarization degree.
However, in the case of the in-liquid polarization, the polarization degree of the piezoelectric body cannot be measured during polarization. This is because the vibration of the piezoelectric body placed in the liquid is damped due to the liquid, and the frequency characteristic cannot be measured. For this reason, conventionally, the in-liquid polarization is the constant-time polarization in which the polarization is carried out for a desired time period. As a result, a problem arises in that the polarization degree cannot be exactly controlled, causing the firing and composition of the piezoelectric body to be dispersed, which results in dispersion of the polarization degree.
In Japanese Patent No. 2656041, a polarization method proposed is in which a piezoelectric constant (for example, electromechanical coupling coefficient K) is measured during polarization, and application of a voltage is stopped when the constant reaches the desired level which is determined by a correlation between the value K obtained immediately after the polarization is stopped and the stable value K obtained after a lapse of time. As a result, dispersion of the polarization degree caused by dispersion of materials and firing conditions is reduced. Accordingly, a piezoelectric body having constant qualities is produced.
According to the above-described method, the piezoelectric constant value is measured during polarization. Accordingly, it is necessary to carry out the polarization treatment in the air. However, in the in-air polarization, discharge occurs at a voltage of about at least 1 kV/mm, so that a high voltage cannot be applied . This causes the problem that a long polarization treatment time is needed to obtain a polarization degree comparable to that by the in-liquid polarization.
Moreover, according to the above-described method, the set level is determined based on the correlation between the value of K obtained immediately after the polarization is stopped and the stable value of K obtained after a lapse of time. However, if the piezoelectric body is aged after the polarization treatment, the polarization degree varies with the value of K. For this reason, if the aging is carried out after the polarization treatment, the set level cannot be determined based on the correlation between the value of K obtained immediately after the polarization is stopped and the stable value of K obtained after a lapse of time.
To overcome the problems described above, preferred embodiments of the present invention provide a method of polarization-treating a piezoelectric body in which a polarization degree comparable to that obtained by in-liquid polarization is obtained in a short period of time by in-air polarization.
Preferred embodiments of the present invention also provide a method of polarization-treating a piezoelectric body in which dispersion of the polarization degrees of respective piezoelectric bodies is reduced, and a target polarization degree is very precisely attained.
According to a first preferred embodiment of the present invention, a method of polarization-treating a piezoelectric body includes the steps of applying a DC voltage to the piezoelectric body for polarization in the air and in an atmosphere with a temperature at least as high as an aging temperature, measuring a polarization degree of the piezoelectric body while the piezoelectric body is polarized, stopping the application of the DC voltage at the time when the measured polarization degree reaches a set level, and aging the piezoelectric body at the aging temperature after the application of the voltage is stopped.
In the case of polarization treatment in the air, a voltage of about at least 1 kV/mm cannot be applied. However, a piezoelectric body can be polarization-treated at a higher temperature as compared with the polarization in a liquid. Accordingly, the polarization is conducted at a low voltage, and a desired polarization degree is attained in a short time. Moreover, since the aging of the piezoelectric body proceeds simultaneously with the polarization, the aging time after the application of the voltage is stopped is substantially shortened. For example, conventionally, for aging in an atmosphere at about 150xc2x0 C., an aging time of about 20 to 30 minutes is required. According to preferred embodiments of the present invention, the aging is performed in only several minutes. That is, the aging time is reduced to about one tenth of the conventional aging time. Consequently, the polarization time and the aging time are greatly shortened. Thus, the overall time required for the polarization treatment is significantly reduced.
Further, the polarization is performed at a relatively low voltage. Therefore, a charge to the piezoelectric body is reduced, and problems such as cracking, chipping, and other common problems, which are caused by the polarization, are eliminated.
As the piezoelectric body is polarized in the air, the frequency characteristic is measured during the polarization. That is, the polarization degree is easily measured. With the polarization degree being measured, the application of the voltage is stopped at the time when the measured polarization degree reaches a set level, and after the application of the voltage is stopped, the piezoelectric body is aged at an aging temperature. Therefore, dispersion of the polarization degree is greatly reduced, and a target polarization degree is very precisely attained. The set level is a value that is determined depending on piezoelectric materials. Further, the set value may be a value determined corresponding to polarization conditions such as polarization temperature, polarization voltage, and other conditions.
FIG. 1 shows the variation of the polarization degree of a piezoelectric body in the process of polarization to aging to ordinary temperature restoration.
As seen in FIG. 1, the polarization degree rises to the maximum during the polarization, and is decreased by the aging, and then is partially restored by the ordinary temperature restoration to become stabilized. The maximum polarization degree xcex94f1 in the polarization and the stable polarization degree obtained after the ordinary temperature restoration have a high correlation.
In the above example, the polarization degree of the piezoelectric body is determined based on the frequency difference xcex94f between the resonance frequency fr and the anti-resonance frequency fa. A piezoelectric constant such as an electromechanical coupling coefficient K, a center frequency, or other suitable piezoelectric constant is used to determine the polarization degree.
FIG. 2 shows a correlation between the maximum polarization degree xcex94f1 in the polarization and the stable polarization degree xcex94f2 after the ordinary temperature restoration. The correlation was determined under the following conditions:
piezoelectric body: PZT block (thickness 8 mm)
polarization voltage (inter-electrode voltage):8.7 kV
polarization temperature: 200xc2x0 C.
aging temperature: 200xc2x0 C.
As seen in FIG. 2, xcex94f1 and xcex94f2 have a high correlation. In this example, xcex94f1 and xcex94f2 are proportional to each other.
Preferably, the set level for stopping the application of the DC voltage is determined according to preferred embodiments of to the present invention. In particular, the set level is determined based on a correlation between the polarization degree of the piezoelectric body obtained immediately before the application of the DC voltage is stopped, and the stable polarization degree of the piezoelectric body obtained after the application of the voltage is stopped so that the piezoelectric body is aged and the temperature of the piezoelectric body is restored to an ordinary temperature. The polarization degree during polarization and the polarization degree (called a residual polarization degree) after aging and ordinary temperature restoration have a high correlation as seen in FIG. 2. The polarization degree during the polarization is determined from a target residual polarization degree by utilization of the correlation. The application of the voltage is stopped at the time when the polarization degree during polarization reaches the calculated polarization degree.
According to the above method, the polarization degree is very precisely controlled to be a target, and dispersion of the polarization is greatly reduced.
According to a second preferred embodiment of the present invention, a method of polarization-treating a piezoelectric body is provided in which the polarization degree of the piezoelectric body is measured while the piezoelectric body is polarization-treated, variations of the polarization degree are expressed as a characteristic formula based on measured polarization degrees, the time required for the polarization degree expressed by the characteristic formula to reach a set level is calculated, and the application of the DC voltage is stopped at the time when the polarization time reaches the calculated time. That is, in the case where piezoelectric bodies have a constant composition, variations in the polarization degrees obtained after the polarization is conducted to some degree are substantially constant. Thus, the time required for a polarization degree to reach a set level is predicted by utilization of the variations. Therefore, the application of the voltage is stopped when the polarization time reaches the predicted time.
According to this method, the measurement of a polarization degree, the expression as a characteristic formula, the calculation of the time required to reach a set level, and other factors are carried out for the respective piezoelectric bodies as the polarization is being attained. In the method according to the second preferred embodiment of the present invention, only the polarization time at a time point near the completion of the polarization is measured, in contrast to the method according to the first preferred embodiment of the present invention in which the polarization degree at the completion of the polarization is measured. Therefore, advantageously, over-polarization, which will be caused by measurement lag, is prevented, and the polarization can be very precisely controlled.
If a thick piezoelectric ceramic substrate such as a block is polarized in a high temperature atmosphere, current flowing through the piezoelectric body is increased over time in some cases. Regarding the current increase, as the polarization proceeds, the internal orientation of the crystal proceeds in the electric field direction, and the insulation resistance of the piezoelectric body are reduced over time. The reduction of the insulation resistance causes the increase of the current value. The current increase during the polarization is presented more distinctly as the temperature increases. With increased current, a voltage drop is generated in the current limiting resistors which are provided for preventing over-current and are connected in series with the respective piezoelectric bodies, so that the voltages applied to the piezoelectric bodies are reduced. The reduction of the voltages causes problems in that the polarization rates of the piezoelectric bodies W1 to Wn are reduced, and a desired polarization degree cannot be obtained.
Accordingly, in the case of a piezoelectric body having the characteristic that current flowing through the piezoelectric body during polarization is increased, for the application of the DC voltage, a voltage drop in a current limiting resistor is calculated based on a current value flowing through the piezoelectric body, and the voltage drop is added to an initial applied voltage.
That is, an applied voltage is determined according to the following calculation expression:
applied voltage=initial voltage+current valuexc3x97current limiting resistance
By keeping constant the voltage applied to each piezoelectric body at any time, dispersion of the polarization degrees of the piezoelectric bodies, caused by dispersion of the applied voltages, is greatly reduced. According to this method, the polarization condition (voltage) is kept constant in addition to the control of the polarization degree. Therefore, dispersion of the polarization degree is further reduced.
Other features, elements, characteristics and advantages of preferred embodiments of the present invention will become apparent from the detailed description of the preferred embodiments of the present invention with reference to the attached drawings.